docs: add `site_url`

[michalsn]

Description This PR:

• adds site_url, to make navigation.instant feature work
• removes navigation.instant.prefetch feature because it's available only for sponsors of the MkDocs project

Checklist:

• [x] Securely signed commits
• [ ] Component(s) with PHPDoc blocks, only if necessary or adds value
• [ ] Unit testing, with >80% coverage
• [ ] User guide updated
• [ ] Conforms to style guide

[kenjis]

Oh, the heart mark means sponsor only. https://squidfunk.github.io/mkdocs-material/setup/setting-up-navigation/#instant-prefetching